The Washington University facility has a VG ZAB-T four-sector tandem mass spectrometer equipped with an extended array detector. The purpose of the research is to develop this unique instrument for studies of biomolecular structure. Subject molecules are peptides, complex lipids, and carcinogen-modified DNA. Currently under investigation is a pulsed FAB ionization technique for use in conjunction with the array detector for analysis of compounds which do not produce strong ion currents under continuous FAB ionization conditions. The approach used here is to modify an existing FAB probe to permit mechanical chopping of the incident fast atom beam. Preliminary experiments show that this approach is sufficiently feasible to justify further development.